At the Royal Court in August 1970, McCowen was cast to play the title role in Christopher Hampton's sophisticated comedy, The Philanthropist. If a philanthropist is literally someone who likes people, McCowen's Philip was a philologist with a compulsive urge not to hurt people's feelings – the inverse of Molière's The Misanthrope. Following enthusiastic reviews the production played to packed houses and transferred to the Mayfair Theatre where it ran for a further three years, making it the Royal Court's most successful straight play. McCowen and his co-star Jane Asher went with it to Broadway in March 1971 where he won the 1971 Drama Desk Award for Outstanding Performance.

Two years later, again at the Mermaid, McCowen gave a portrayal of the British poet Rudyard Kipling in a one-man play by Brian Clark, performed in a setting that exactly matched Kipling's own study at Bateman's (his Jacobean rustic haven in Sussex) "and turning", as Michael Billington wrote, "an essentially private man into a performer." McCowen appeared in the play on Broadway and on television for Channel 4.

Television roles included the BBC's four-part adaptation of J. B. Priestley's Angel Pavement (1958), and his one-man stage performance of The Gospel According to Saint Mark, transferred to television by Thames for Easter 1979.[8] He appeared in the BBC Television Shakespeare series as Malvolio in Twelfth Night and as Chorus in Henry V. In 1984 and 1985 McCowen starred in the ten episodes of the shortlived television series Mr Palfrey of Westminster as a "spy catcher" working for British intelligence under the direction of a female boss (played by Caroline Blakiston).